How will Google make money from Android?
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A) Knowing the make-up of your daily online life
B) Presenting ads that are relevant based on that profile
Android keeps you inside the Google ecosystem. Google search, Google mail, Google docs, Google RSS reader, Google shopping. Each of these utilities feed the core purpose of knowing you, which is beneficial to Google's bottom line. The better they know their users, the better targeted the ads can be, and the better they can convert.
With Android, Google accelerated/is accelerating the pace of smartphone adoption (more smartphone = more internets = more Google = more Ads) and at the same time putting their services in front of the smartphone vendors for a undisclosed, but possibly tiny, fee per device. Enabling Google Search as default but not requiring it (as it's been the case of many Android smartphones with Bing by default in the US).
Note1: Android is free. Google Core-Android-Apps are not: Gmail, Market, and so on
Note2: Maybe even the fee is not a monetary one. It could be the option of selling the smartphone as a 'Android' phone (with is trademarked), using the Android logo, etc.
[1] http://techcrunch.com/2011/03/25/search-googles-castle-moat/